1. Field of the Invention
The invention relates to high pressure liquefier operations. More particularly, it relates to improved energy efficiency in such operations.
2. Description of the Prior Art
Many processes, both once-through and recycle types, have been used to liquefy air separation products, namely nitrogen, oxygen and argon. Around the middle of this century, processes were employed in which feed air to an air separation plant was compressed to as high as 3,000 psig in piston type, positive displacement reciprocating compressors. The high pressure air was dried and cooled in shell and tube, or spiral-wound, heat exchangers and expanded through reciprocating, positive displacement, work extraction expanders to produce the refrigeration necessary for producing air separation liquids. Such high pressure operation offered significant liquefaction cycle thermodynamic efficiency advantages. However, the heat exchange equipment employed was bulky and expensive, and the reciprocating machinery was complex and costly, both from an investment and maintenance viewpoint.
In the late fifties, viable low pressure, multi-stage centrifugal compressors, radial-inflow turboexpanders and compact, cost-effective brazed aluminum heat exchangers became commercially available. Low pressure recycle nitrogen processes were employed to utilize this new equipment for the production of refrigeration to liquefy air separation products. The low aerodynamic efficiency of said machinery and the thermodynamic disadvantage of low pressure operation resulted in liquefaction systems whose energy efficiency was, at times, lower than that of the high pressure systems they replaced. However, investment and maintenance requirements were lower. By the early eighties, steady advances in working pressure and maximum size availability of brazed aluminum heat exchangers, improvements in aerodynamic efficiency of centrifugal compressors, and the commercial availability of multi-stage, centrifugal, high pressure, nitrogen recycle compressors with matching cryogenic turboexpander/booster assemblies were utilized in both recycle and single pass liquefaction cycles with maximum pressures as high as 770 psig. Energy efficiency was significantly better for these newer designs than for the earlier, low pressure turbomachinery-based systems. At the present time, most air separation liquids are manufactured by liquefiers of such improved design.
Typical configurations of the present type of nitrogen liquefier is illustrated in the Hanson et al patent, U.S. Pat. No. 4,778,497. As shown therein, first feed nitrogen is supplied to the suction of a three or four stage recycle compressor from the discharge of the feed compressor supplied with low pressure nitrogen from an air separation plant. Additional feed is often supplied as warmed vapor from the high pressure column in the air plant. The nitrogen recycle compressor pumps this feed and the returning recycle nitrogen stream from the liquefier cold box from a pressure of typically about 80-90 psia to about 450-500 psia. The total recycle compressor discharge stream is further compressed to about 700 psia by warm and cold turbine boosters arranged in parallel as shown in the Hanson et al patent. For this liquefaction cycle arrangement, parallel rather than series arrangement of the boosters results in the most advantageous dimensionless aerodynamic performance parameters for the booster compression stages. The high pressure stream exiting the boosters is successively cooled in the cold box brazed aluminum heat exchangers and divided between the warm turbine, cold turbine and the product stream. The exhaust from both turbines is warmed in the heat exchange system and returned to the suction of the recycle compressor.
In 1985, large brazed aluminum heat exchangers with working pressure capability of 1,400 psig became available. For a number of reasons, the nitrogen liquefaction process described above is not able to benefit from the thermodynamic advantages of operating at this higher pressure level. With both turbines operating at a pressure ratio of about 8, e.g. 700 psia to 88 psia, the sum of the temperature drop across the two machines equals the total temperature range from ambient to saturated vapor temperature at the cold turbine exhaust. Increasing the inlet pressures of the turbines without increasing their outlet pressure would increase the temperature drop across the machines beyond that which can be efficiently used by the process. Thus, temperature mixing losses and/or two phase exhaust from the cold turbine would develop. Also, the pressure ratio across a single stage radial inflow turboexpander cannot be increased much beyond 8 because of aerodynamic design constraints. These problems could be avoided by increasing both the inlet and outlet pressures of the turbines proportionately to maintain the pressure ratio across them fixed at about 8. At a 1,400 psia turbine inlet pressure, exhaust pressure of the turbines and inlet pressure to the recycle compressor would be about 175 psia. The cold turbine exhaust temperature could not be lower than the saturation temperature of 107.degree. K. at 175 psia which, in turn, would result in excessively high temperature and enthalpy of the supercritical product stream entering the flash separator, exported to the air plant, or passing to the subcooler for subsequent delivery to storage. The overall efficiency of the system is hurt by this reduction in the proportion of total liquefaction refrigeration that is provided by direct heat exchange contact with the turbine exhaust streams. In addition, increasing the exhaust pressure of the cold turbine and suction pressure of the recycle compressor above the operating pressure of the high pressure column in the air separation plant prevents direct transfer of either cold or warmed vapor from this column to the suction circuit of the liquefier. While various means for avoiding this problem can be attempted, they all add appreciable cost and complexity to the plant. As a result, therefore, the liquefaction processes operating at peak cycle pressures of 700-800 psia and currently used widely to liquefy nitrogen and air are not well suited for operating at higher peak cycle pressures.
The Dobracki patent, U.S. Pat. No. 4,894,076, discloses a turbomachinery-based, recycle nitrogen liquefaction process designed to take advantage of the commercially-available high working pressure brazed aluminum heat exchangers. As indicated in Table I, thereof, the patented process has a claimed energy efficiency advantage of about 5% compared to typical commercial liquefiers. The patented process uses three radial-inflow turboexpanders to span the temperature range from ambient to saturated vapor exhaust of the cold turbine. The warm turbine, taking aftercooled recycle compressor discharge gas at 489 psia as feed, discharges at recycle compressor suction pressure of 91 psia and 192.degree. K. It provides all of the refrigeration required by the process down to the 200.degree. K. temperature level. The remaining recycle compressor discharge gas is boosted from 490 psia to maximum cycle head pressure of 1,215 psia by two centrifugal compressor wheels absorbing power delivered by the three gas expanders. After cooling to 200.degree. K. in the heat exchange system a portion of this stream is directed to the intermediate gas expander where it expands to 480 psia and 155.degree. K. This machine provides process refrigeration between 200.degree. K. and 155.degree. K. The cold turboexpander is fed exhaust gas from the intermediate expander blended with a small trim stream of recycle compressor discharge gas which has been cooled in the heat exchange system to the same temperature. The cold expander exhausts at 94 psia at, or close to, saturated vapor. It provides refrigeration between 155.degree. K. and 99.degree. K. The turbine exhaust stream after being warmed in counter-current heat exchange with incoming feed stream returns to the recycle compressor suction. The liquid, or dense fluid expander, expands the cold, supercritical product nitrogen stream from 1,206 psia to 94 psia for further heat content reduction before export to the air separation plant as refrigeration supply for production of subcooled liquid products. While the patented process is disclosed as having an overall energy efficiency better than the prior art by about 5%, there nevertheless remain several deficiencies and disadvantages that are desired to be overcome to further advance the liquefier art.
The power requirement of the Dobracki patent process is 2.3% greater than that of the invention herein described and claimed. Two factors contributing to this circumstance are that its reported cycle pressure of about 1,200 psia is lower than the currently preferred 1,400 psia level of the subject invention, and, secondly, the power generated by the liquid turbine is not recovered to accomplish useful work. Furthermore, the cycle is more complicated because it uses three nitrogen gas turbines and one liquid turbine with incremental investment and maintenance costs being high because of the use of four machines as compared to the simpler scheme of the subject invention involving two gas turbines and one liquid turbine.
The cycle arrangement of the Dobracki patent will be seen to preclude achieving the thermodynamic advantage theoretically available from increasing process head pressure to 1,400 psia, the maximum working pressure capability of today's brazed aluminum heat exchangers, or desirably up to 2,500 psia.
It will thus be seen that it would be highly desirable in the art to have high pressure liquefier processes capable of advantageously employing heat exchangers with working pressure capability up to 1,400 psia. It should also be noted that, in many instances where the liquefier is integrated with an air separation plant, it would be advantageous to have the flexibility of lowering the cold turbine exhaust pressure and recycle compressor inlet pressure to permit exporting either or both warmed and cold nitrogen vapor from the air separation plant's high pressure column without compression, as feed to the liquefier. Modern air separation plants with structured packing-filled distillation columns are being designed with high pressure nitrogen column pressures as low as 68 psia. The process of the Dobracki patent does not have the flexibility of operating at a recycle compressor suction pressure this low. If it were attempted, either very large liquid content would develop in the cold turbine exhaust, or large temperature mixing losses would occur between the heat exchanger zones. This problem could be resolved by operating at a maximum cycle pressure of about 900 psia, but this would result in a significant reduction in cycle energy efficiency.
It is an object of the invention to address these various problems in the art so as to provide an improved high pressure liquefier process and system capable of utilizing high pressure heat exchangers and of achieving significant process energy savings over current practices in the art.